Use approved_schedule property for detecting ScheduledForPublishLock

We only need to know about the existence of a scheduled revision,
and not the revision itself.

This method is used by 'PagePermissionTester.can_unpublish', which
in turn is used by the 'Unpublish' button in the page listing 'more'
button. As a result, this method is run on every single item in the
listing.

We already have annotate_approved_schedule to optimise the display
of the 'scheduled' status tag (for pages). Use the approved_schedule
property so we can reuse that annotation instead of performing an
N+1 query.

wagtail/admin/tests/pages/test_explorer_view.py

@@ -1284,7 +1284,7 @@ class TestInWorkflowStatus(WagtailTestUtils, TestCase):
         # Warm up cache
         self.client.get(self.url)
 
-        with self.assertNumQueries(47):
+        with self.assertNumQueries(44):
             response = self.client.get(self.url)
 
         self.assertEqual(response.status_code, 200)

wagtail/models/__init__.py

@@ -642,7 +642,7 @@ class DraftStateMixin(models.Model):
 
     def get_lock(self):
         # Scheduled publishing lock should take precedence over other locks
-        if self.scheduled_revision:
+        if self.approved_schedule:
             return ScheduledForPublishLock(self)
         return super().get_lock()